The present invention relates to an improved sub-surface grouting material for use as at least one of a thermally conductive fill material and an electrically conductive fill material, as well as new and useful applications for the use of conventional Grout 111.
Grouts comprised of various mixtures of bentonite clay, sand, water, and the like, are commonly used to fill the void, and poorly conductive, spaces in boreholes/wells, after tubing has been inserted into the boreholes/wells, so as to provide at least one of a thermally conductive and an electrically conductive means with the surrounding ground. Thermally conductive grouts are typically utilized to fill the voids in the wells/boreholes of geothermal heat pump systems so as to promote heat transfer to and from the heat transfer fluid circulating within closed-loop tubing from and to the surrounding ground, depending on whether the heat pump system is operating in the cooling or the heating mode. A special, highly thermally conductive, cementitious grout, commonly referred to as “Grout 111”, was developed for this purpose, as more fully described in U.S. Pat. No. 6,251,179 B1 to Allan, the disclosures and claims of which are fully incorporated herein by reference. This grout has improved thermal conductivity, improved bond strength, and decreased permeability, all of which are desirable properties for heat transfer purposes. Grout 111 attains a thermal heat transfer rate of 1.4 Btu/hr. ft. degree F.
Also, U.S. Pat. No. 4,912,941 to Buichi teaches a method of extracting thermal energy from the earth via using a thermally conductive grout mixture of water and cement, together with one or more of a siliceous gel and finely divided metal powder. The thermally conductive grout mixture taught by Buichi is one or more of a siliceous gel and a finely divided metal powder, preferably silver and/or copper and/or aluminum powder. The substance, in fluid form, is used for pressure grouting into the crevices of artificially fissured rock (created by blasting and flushing), where it solidifies into a heat conductive plug within the (see U.S. Pat. No. 4,912,941 to Buchi, column 1, line 40 through column 2, line 22).
However, it is important to note that the thermally conductive grout mixture taught by Buchi is not for geothermal heating/cooling applications, and is not for the purpose of enhancing the sub-surface heat exchange line in a geothermal heat pump system application. In a geothermal heat pump system application, whether a water-source system or a direct expansion/direct exchange system (which systems are well understood by those skilled in the art) the heat transfer fluid is circulated within a supply and a return pipe/tube, and it is critical that the grout surrounding the sub-surface heat transfer fluid containment piping/tubing be as thermally conductive as possible. To the contrary, Buchi expressly teaches that it is important to lose as little heat as possible during the ascent of the hot transmission medium in the return pipe, which pipe, at least in its upper reaches (upwards of 1,500 meters deep) is thermally insulated via a special steel, asbestos-cement, or synthetic resin (see U.S. Pat. No. 4,912,941 to Buchi, column 2, line 43 through column 2, line 48). Further, Buchi teaches how to solely extract heat, principally via steam or hot water (see column 2, lines 49 through 56) from depths of more than 1,500 meters to preferably 5,000 meters (see column 1, lines 52 through 54). Buchi' design is never intended for the earth to act as a heat sink, as would be the case in a reverse-cycle geothermal heat pump system, and is not intended for applications less than 1,500 meters deep (typically geothermal heat pump systems operate at depths down to about 300 feet deep).
Thus, the teachings of Buchi, which require injecting materials into artificially created fissures in rock, at depths of at least 1,500 meters, so as to solely supply very hot water or steam, together with an insulated return line, would not be apparent to those working with reverse-cycle geothermal heat pump systems at typical depths of 300 feet, or less, which typically utilize the ground as both a heat source and a heat sink, which require a heat conductive fill material to surround the sub-surface heat transfer line for mostly the entire length of the sub-surface heat transfer containment tubing/piping, and which do not intentionally create, flush out, and re-fill, lateral fissures in deep rock. Further, while Buchi teaches the use of water and cement mixed with a preferable silver, copper, or aluminum powder, Buchi overlooks the facts that the surface of silver and copper quickly oxidize, which oxidized surface inhibits heat transfer, that a powdered metal will result in almost a maximum oxidation impairment, and that aluminum does not mix well with cement.
Geothermal heat pump systems are commonly comprised of “water-source” systems, which exchange heat to and from the ground to water, then to a refrigerant, and finally to interior air, and of Direct eXchange, also commonly referred to as “Direct eXpansion” (herein abbreviated as “DX”) systems, which exchange heat to and from the ground to a refrigerant, and then to the interior air. Since DX systems have one-third fewer heat transfer steps and do not require a water circulator pump, DX systems are typically more efficient than water-source heat pumps. As mentioned, the specific differences between water-source heat pumps and DX heat pumps are well understood by those skilled in the art.
The sub-surface heat transfer tubing utilized in geothermal heat pumps may be installed in at least one of a hole/borehole/well, a trench, and a pit. A hole/borehole is typically drilled, and may reach depths of about 300 feet. A trench is typically dug via a trencher to a depth of about 4 to 5 feet. A pit is typically dug via a front end loader or an excavator to a depth of about 4 to 5 feet. However, the fill material immediately surrounding the heat transfer tubing in at least one of a trench and a pit type installation has historically been one of natural soil and/or sand and/or powdered limestone.
Most common grouts used for heat transfer purposes in conjunction with geothermal heat pump applications are comprised of various mixtures of bentonite clay, sand, and water, absent cement. The absence of cement makes the grouts easier to pump via less expensive pumping equipment. Such common clay-based grouts without cement typically attain heat transfer rates of only 0.69 to 1.2 Btu/hr. ft. degree F., with the heat transfer rate being adversely affected as the grout is dried out via heat pump system operation in the cooling mode. Dried, clay-based, grouts tend to significantly loose heat transfer abilities and also tend to shrink and pull away from sub-surface geothermal heat transfer tubing, leaving non-heat conductive void areas. For example, a moist clay would have a heat transfer rate in the range of 0.64 Btu/hr. ft. degree F., while a dry bentonite clay would have a heat transfer rate in the range of 0.1 Btu/hr. ft. degree F.
While the aforesaid Grout 111 was developed for geothermal heat transfer purposes, it was, in reality, primarily developed for use in conjunction with water-source geothermal heat pump purposes, as evidenced via the disclosures of the aforesaid Grout 111 at U.S. Pat. No. 6,251,179 B1. Specifically, this is because water-source geothermal heat pump systems virtually always utilize a plastic, high density, polyethylene (“HDPE”) piping within which to circulate a water and/or a water/antifreeze fluid mixture for sub-surface heat exchange purposes. This plastic tubing is virtually impervious to common underground conditions, such as sulfur water, etc., that may be corrosive to copper tubing, which copper tubing is virtually always used to circulate a refrigerant fluid for sub-surface DX system heat exchange purposes.
Therefore, since the claims of the said Grout 111 patent are intended for use with HDPE piping in a water-source geothermal heat pump system application, they do not in any way restrict the type of water to be utilized in the grouting formula, other than a common sense reference that the water should be potable, not having excessive impurities, but with absolutely no mention being made of pH level requirements, (see U.S. Pat. No. 6,251,179 B1 to Allan, column 6, lines 30 through 34). However, this lack of pH level restriction, in a DX system application, could be catastrophic, resulting in sub-surface refrigerant transport tubing deterioration and leaks, as sulfur water, water with a pH level below 5.5, and water with a pH level above 11, and the like, can be corrosive to copper. Thus a means to improve the water formula for inclusion in a Grout 111 mix would be preferable for a DX system application.
Also, a means to improve the heat conductivity for a Grout 111 mix would additionally be preferable for both a DX system and a water-source system geothermal heat pump application. Both DX and water-source geothermal heat pump systems commonly utilize U loops in vertically oriented wells/boreholes, to circulate a heat exchange fluid into and out of the wells/boreholes, while exchanging heat with the naturally occurring sub-surface temperatures. The U loops in a DX geothermal heat pump system are typically comprised of copper tubing, which transport a refrigerant heat exchange fluid. The U loops in a water-source geothermal heat pump system are typically comprise of plastic HDPE tubing, which commonly transport a water/anti-freeze heat exchange fluid.
Additionally, common bentonite clay-based grouts, absent any cement, are typically utilized for electrically conductive purposes to fill the void areas surrounding holes drilled for grounding rods, such as the grounding rods used by utility companies, radio transmission towers, and the like. Grounding rods are typically comprised of a single, solid, metal rod, (not U bends of tubing, as in the case of geothermal heat pump systems) with the metal typically comprised of copper, or the like. Common clay-based grouts are typically utilized for such electrically conductive purposes because the industry as a whole has been more concerned with simply filling the void spaces in grounding rod boreholes than in developing a method to both fill the borehole voids and to simultaneously increase electrical conductivity, which would be preferable. Increasing the electrical conductivity of grouts used to fill grounding rod boreholes will increase the protective qualities of the grounding rods, helping to better insure excessive and undesirable electrical currents, such as from lightening strikes for example, are fully and completely directed into the ground and away from buildings, people, computers, transmitters, equipment, and the like. Further, it would be preferable to utilize a cementitious grout, with a low moisture permeability, to surround metal grounding rods so as to at least one of help avoid poor conductivity during periods when clay-based grouts may tend to dry out, and so as to additionally help to protect the metal grounding rods from premature deterioration caused by soil conditions potentially corrosive to metals.